*3.2. E*ff*ect of Tube Pressure*

The Hyperloop Alpha documents recommended an ideal tube pressure of 1/1000 atm for operating the Hyperloop systems. In this section, to verify this factor, different tube pressures were applied for the pod speed of 300 m/s with a BR of 0.36 and a pod length of 43 m. Figure 10a illustrates the changes in drag and drag coefficient with the tube pressure. As shown in the sub-figure, aerodynamic drag linearly increases with tube pressure. With increasing tube pressure, pressure drag becomes more dominant, whereas the variation of friction drag is minor. The tenfold increase of tube pressure has increased the total drag by more than nine times. Therefore, it is better to maintain a lower tube pressure to have a smaller drag. The variation of drag coefficient is inversely proportional to the flow density, reference area, and the square of operating speed. As mentioned earlier, the cross-sectional area and operating speed were fixed to investigate the effect of tube pressure. Hence, in this study, the change of drag coefficient with respect to tube pressure is mostly affected by the density of air, which is calculated by ρ = *Pt*/*RT*, where *Pt* is the tube pressure, *R* is the gas constant, and *T* is the temperature. The higher tube pressure causes higher density, resulting in a larger *Re* and a marginal decrease in the drag coefficient. Figure 10b shows the proportion of pressure and friction drag in the total drag. Note that tube pressure has only a small influence on the proportion of the drag components to the total drag.

(**b**) Ratio of component drag to total drag

**Figure 10.** Effect of tube pressure: (**a**) Variation of drag and drag coefficient with respect to the tube pressure. (**b**) Ratios of the pressure and friction drags to the total drag.
